With the increasing integration of ICs, hundreds of thousands of MOS transistors can be formed on a silicon substrate with an area of 1.about.2 cm.sup.2 in an advanced VLSI process. In order to operate the transistors independently, each transistor must be isolated to prevent shorting. The process is called the "isolation process".
A conventional isolation process comprises the following steps. First, an oxide and a hard mask are formed on a semiconductor substrate in sequence. Then, a window is defined at the determined STI site by using photolithography and etching techniques. Subsequently, the exposed substrate within the window is removed to form an STI trench. The detailed process is illustrated in FIGS. 1A.about.1C.
First, referring to FIG. 1A, a semiconductor substrate 100, such as a silicon substrate, is provided. Then, an oxide 110 (e.g. a silicon dioxide layer) and a hard mask layer 120 (e.g. a nitride layer) are formed on the semiconductor substrate 100 in sequence. Then, a photoresist pattern 130 with a opening 140 exposing the hard mask 120 at in a predetermined STI site is formed on the hard mask 120 by photolithography techniques.
Next, referring to FIG. 1B, the exposed hard mask 120 and the underlying oxide layer 110 and semiconductor substrate 100 within the opening 140 are etched to form an etching window 140' exposing the semiconductor substrate 100 by means of photolithography techniques.
Finally, referring to FIG. 1C, the photoresist layer 130 is removed, and the exposed semiconductor substrate 100 within the opening 140' is etched out by using the hard mask 120 as an etching mask, thus an STI trench 150 is formed. The STI trench 150 can be further gap-filled by an insulating material, though this step is not detailed here.
It is noted that the corners of the STI trench 150 are very sharp, therefore the STI channel obtained after gap-filling with an insulating material will make the insulating layer around the corners thinner than at other sites. Hence, leakage current or double hump may be apparent during operation, thus establishing a parasitic electric field.
In order to address the drawback of the conventional STI process described above, it is necessary to develop a novel STI process to forming top-rounded trenches for isolation.